1. Field of the Invention
This invention relates to a rear wheel turning system for a vehicle which turns the rear wheels of the vehicle in response to turning of the front wheels through a mechanical system and a hydraulic system and to a method of neutral adjustment of the rear wheel turning system for adjusting the rear wheel turning system so that when the mechanical system is in the neutral position where it holds the rear wheels straight ahead, the hydraulic system is in the neutral position where it doesn't apply hydraulic pressure to the mechanical system and when the hydraulic system is in the neutral position, the mechanical system is in the neutral position.
2. Description of he Prior Art
There has been known a four-wheel steering system in which the rear wheels are turned in response to turning of the front wheels. As the system for turning the rear wheels in such a four-wheel steering system, there has been known a rear wheel turning system which, as disclosed for instance in Japanese Unexamined Patent Publication No. 1(1989)-273772, comprises a mechanical system for displacing a rear wheel turning shaft to turn the rear wheels and a hydraulic system for assisting the mechanical system in displacing the rear wheel turning shaft. The mechanical system comprises an input shaft into which the front wheel turning amount is input, an output rod member which receives input from the input shaft and makes a stroke in its axial direction, a yoke assembly which is connected to the output rod member and is supported for swinging motion, and a displacement transmission means which is connected to the output rod member, the rear wheel turning shaft, and the vehicle body and transmits the stroke of the output rod member to the rear wheel turning shaft. The stroke of the output rod member for a given rotating angle of the input shaft is controlled according to the swinging angle of the yoke assembly. The hydraulic system comprises a power steering means which is connected to the rear wheel turning shaft and assists the mechanical system in displacing the rear wheel turning shaft, and a hydraulic switching valve which is connected to the displacement transmission means and controls the power steering means. That is, the mechanical system and the hydraulic system are connected with each other by way of the displacement transmission means and the hydraulic switching valve.
In such a rear wheel turning system, the stroke of the output rod member, i.e., the amount of displacement of the rear wheel turning shaft, is determined depending on the rotating angle of the input shaft and the swinging angle of the yoke assembly in the mechanical system and the power steering system assists the mechanical system in displacing the rear wheel turning shaft by the amount.
Accordingly, in order to accurately control turning of the rear wheels, the mechanical system and the hydraulic system must be connected with each other so that one of the hydraulic system and the mechanical system is never in the neutral position but the other is in the neutral position.
When connecting the mechanical system and the hydraulic system, the mechanical system (the input shaft and the yoke assembly) is first adjusted to the neutral, and then the displacement transmission means is connected to the mechanical system (that is, to the output rod member and the rear wheel turning shaft). Thereafter, the hydraulic switching valve is incorporated and the supporting portion at which the displacement transmission means is supported by the vehicle body is determined, with the hydraulic pressure applied, so that the neutral of the mechanical system and that of the hydraulic system conform to each other, whereby deviation from the neutral of the hydraulic switching valve itself due to inaccuracy in machining and the like can be cleared and slip of the neutrals of the systems relative to each other can be absorbed.
Generally the rear wheel turning system is provided with a centering spring on the rear wheel turning shaft, and when the hydraulic system loses hydraulic pressure or when the mechanical system fails and the hydraulic system is drained to release hydraulic pressure from the power steering system, the centering spring holds the rear wheel turning shaft in the neutral position where it holds the rear wheels straight ahead. That is, in the case of failure in the systems, the centering spring provides so-called fail-safe function.
When the steering wheel is turned with the rear wheel turning shaft in the neutral position and the hydraulic switching valve is operated to increase the hydraulic pressure acting on the rear wheel turning shaft, the rear wheels are not turned and held in the neutral position until the hydraulic pressure acting on the rear wheel turning shaft exceeds the force of the centering spring. That is, there is a dead zone where the rear wheels are not turned even if the front wheels are turned.
The supporting portion of the displacement transmission means at which the displacement transmission means is connected to the vehicle body may be adjusted to the neutral position by measuring the stroke of the rear wheel turning shaft (output shaft) in response to displacement of the supporting portion with the hydraulic system applied with hydraulic pressure, thereby finding the dead zone where the output shaft is not displaced in response to displacement of the supporting portion, and fixing the supporting portion in the position corresponding to the center of the dead zone. In this manner the rear wheel turning system can be adjusted so that one of the hydraulic system and the mechanical system is never in the neutral position but the other is in the neutral position.
However when the stroke of the rear wheel turning shaft (output shaft) in response to displacement of the supporting portion is measured, there is produced hysteresis in the stroke characteristics of the rear wheel turning shaft and the width of the dead zone fluctuates due to existence of the centering spring of the rear wheel turning shaft and coned disc springs on opposite ends of the centering spring and due to mechanical backlash such as clearance between engagement portions, backlash of gears, and the like which is inherent to the mechanical system.
Accordingly, in accordance with the method described above, the neutral position cannot be determined with a high accuracy though it can be determined that the neutral position is to be within a certain range.
When the neutrals of the hydraulic system and the mechanical system conform to each other, the maximum displacement of the rear wheel turning shaft when the input shaft is rotated from its neutral position in the clockwise direction is equal to that when the input shaft is rotated from its neutral position in the counterclockwise direction so long as the rotational angles are the same.
In other words, by adjusting the position of the supporting portion so that the leftward maximum displacement of the rear wheel turning shaft is equal to the rightward maximum displacement of the rear wheel turning shaft, the neutrals of the hydraulic system and the mechanical system can conform to each other.
Further the narrower the dead zone is, the more accurately the neutral position can be determined. Accordingly there is a demand for the rear wheel turning system having a narrower dead zone.